Nucleolin is a protein that is expressed at elevated levels in transformed cells. Tumor cells have been shown to present nucleolin on the cell surface as well as expressing it in the cytoplasm and nucleus. Nucleolin plays multiple roles in the cell and is involved in ribosome biogenesis, cell growth, and DNA replication.
Aptamers are about 60˜80 mers of synthetic ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) oligonucleotides which are known to be discovered by the process called systematic evolution of ligands by exponential enrichment (SELEX) based on high affinity and specific molecular fit with their targets of interest.
Aptamer have recently preferred to be applied for diagnosing and treating cancers as imaging target agents rather than monoclonal antibodies due to the following characteristics: inexpensive, efficient and rapid for production, highly stable for long-term storage, versatile molecules that can be easily modified with imaging probe, small size (8-15 kDa) resulting low immune risk and better penetration into target tissues in vivo, and high affinity molecular probe.
A large number of aptamers targeting cancer-related proteins, such as Wilim's tumor protein 1(WT1), transcription factor 1(TCF-1), human epidermal growth factor receptor 3(HER-3), prostate-specific membrane antigene (PSMA), tenascin-C, nucleolin, pigpen and vascular endothelial growth factor (VEGF), have been developed to target and image cancers.
Some chemical modifications on the ribose backbone of aptamer nucleotides using 2′-amino or 2′-fluoro pyrimidines have been in situ and in vitro studied with the existing aptamers to be resistant to nucleases, more capable of transfer across membranes or more capable of specific binding to the target of interest, but they influence the structure of aptamers, resulting in the loss of aptamer properties.